This protocol describes a phase 1/2 safety and pharmacokinetic study of escalating doses of AMD-3100 administered by continuous intravenous infusion for 10 days in stable HIV-infected patients with viral load >5000 copies/ml. To be eligible, patients are required to be on no antiretroviral therapy or to be taking a stable regimen of antiretroviral drugs for >4 weeks prior to inpatient admission for study drug administration. The ojectives of the study are to assess the safety, pharmacokinetics, and antiviral activity of AMD-3100 administered for 10 consecutive days by continuous intravenous infusion. Each cohort of 4 patients will receive AMD-3100 at successively higher doses, if the previous dose was well tolerated by at least 3 of the 4 patients in the cohort. Patients will be hospitalized for the duration of the study and continue taking the same antiretroviral drugs (or none) unless the development of drug related adverse events warrants drug discontinuation. AMD-3100 is a member of a new class of "fusion inhibitors", which acts to block entry of HIV into CD4+ cells by interaction with the human CXCR4 co-receptor. Objectives: 1) to determine the safety, pharmacokinetics, and antiviral activity of escalating doses of AMD-3100 administered by continuous intravenous infusion for 10 days in HIV-infected patients on stable or no antiretroviral therapy. 2) to correlate the antiviral activity of AMD-3100 with the co-receptor assay results at baseline and at the end of treatment."